Given the growing world wide elderly population, the incidence of degenerative lumbar disease including low back pain or nerve compression pain is rapidly increasing. Osteoporosis is the most frequent skeletal disease with the highest fracture risk existing for vertebrae. The aging process is accompanied by systemic changes, with the earliest degeneration occurring in the intervertebral discs.
Pedicle screw fixation(PSF) is widely used as the mainstay surgical treatment for degenerative lumbar disease artificially by fixing the damaged spinal level to control movement and thereby reducing the pressure on the nerves. However, studies have shown an increase in disc disease associated with a compensatory increase in motion at adjacent disc levels. Moreover, fusion may cause an unwanted increase in facet load at adjacent levels due to disproportionately higher posterior load transfer.
The normal aging process is accompanied by changes in a lumbar spine with osteoporosis and the earliest degeneration occurring in the intervertebral discs. However, no study relating the effect of property changes in the bony parts and the intervertebral disc of a lumbar spine was found. And biomechanical efficacies of pedicle screw fixation in a lumbar spine with osteoporosis and disc degeneration also need to be elucidated. In this study, a finite element(FE) study was performed to assess biomechanical efficacies of the pedicle screw fixation in a lumbar spine with osteoporosis and disc degeneration. Stiffness changes of the lumbar spine, center of rotation(COR) and range of motion(ROM) at index(operated) and adjacent levels, as well as intradiscal pressures at adjacent level and the ratio between the peak von Mises stress and yield strength of pedicle screws under the physiological loading conditions were investigated.
A previously-validated 3-dimensional FE model of the lumbar spine was used(L1-S1). one-level fusion using posterior pedicle screws with rigid rods was considered as the post-op model. The various degrees of disc degeneration(IVD1∼IVD5) and effect of different possible alterations of the bone quality due to osteoporosis was simulated by adjusting the corresponding material properties. Load was applied at the superior surface of L1 using flexion/extension of 10Nm, axial rotation of 10Nm, and lateral bending of 10Nm with a compressive follower load of 400N. Hybrid protocol was used to study the ROM and intradiscal pressures at the operated and adjacent levels.
Disc degeneration and bone quality due to osteoporosis altered the stiffness and COR of a lumbar spine in flexion/extension, axial rotation and lateral bending. When osteoporosis was simulated, the stiffness was substantially decreased. Compared to the normal disc, the stiffness was reduced along with degrees of disc degeneration. Implantation of PSF decreased ROM at the operative level. Normal+PSF showed the greatest increase in intradiscal pressures at the adjacent level, followed Osteoporotic+PSF, Normal, and Osteoporotic model depending on the loading type. The ratio between the peak von Mises stress and yield strength of pedicle screw remained less than 20% regardless of loading conditions indicating low likelihood of component failure.
Based on these results, in terms of the adjacent segment disease, it can be concluded that the definition of degenerative disc in a normal bone quality model with pedicle screw fixation might be considered as a worst-case scenario.